The driver and other occupants of a cab of a vehicle, such as a tractor for a semitrailer, should be protected or isolated from unnecessary physical disturbances to improve their comfort and safety. Such disturbances as noise, heat and rough rise cause the occupants of a vehicle to tire more readily, thus making them less comfortable and reducing the alertness of the driver to possible hazardous conditions. Vertical movement of the cab due to irregularities in the roadway surface must be expected to some degree, and it has been found that the human body can tolerate an appreciable amount of such vertical movement without pain or discomfort. On the other hand, fore and aft movement in a horizontal plane cannot be tolerated to the same degree by the human body and often results in soreness or at least early fatigue. The disturbing effects of fore and aft movement are especially felt by an occupant of a sleeper berth in the cab and may actually prevent sleep, inasmuch as that occupant derives no benefit from a driver's suspension seat.
Due to the physical characteristics of typical semitrailer vehicles, irregularities in the road surface cause not only vertical movement of the tractor cab but also considerable fore and aft movement. This fore and aft movement may be produced by pitching of the tractor directly induced by road irregularities and often compounded by a reaction to similar pitching action of the attached semitrailer. Fore and aft movement may also be the result of resonant vibration of the frame induced by eccentricity or unbalance of the rolling assembly (i.e. tires, wheels and brake drums) for the vehicle or by resonant pitching of the semitrailer.
Past efforts to achieve significant ride improvement by modifying chassis suspension systems have had limited success. Chassis suspensions having relatively low spring rates are capable of isolating road irregularities to a greater degree than conventional suspension systems, but trailer induced pitch and reduced roll stability become more of a porblem with low spring rate suspensions. Tractor and trailer chassis suspension systems having relatively low spring rates are also more expensive to manufacture, heavier, and cost more to maintain than conventional suspension systems.
Another approach to eliminate the fore and aft movement of a truck cab is to mount the cab on the frame so as to isolate the cab from the frame, and, in particular, from the resonant vibrations of the frame. Since resonant vibration is generally manifested in beam-like flexure about one or more nodal points, it has previously been proposed to mount a truck cab on a frame at a node point near the front of the frame. Thus, no vertical movement is transmitted to the cab from the frame at that mounting. A second mounting for the cab, spaced longitudinally of the frame from the node point, is constructed so as to isolate the cab from at least vertical movement of the frame at the second mounting. The second mounting may be, for example, a loose slide fitting or a resilient, spring mounting.
The general principles of cab isolation presented above are outlined in a pamphlet entitled "Truck Ride Guide" by William F. LeFevre, published in 1967 and copyrighted by Rockwell-Standard Corporation of Detroit, Michigan. Specific embodiments of these general principles are described and illustrated in the De Haan U.S. Pat. No. 3,010,757, the Fisher U.S. Pat. No. 3,276,812 and the Harbers U.S. Pat. No. 3,361,444, for example.